Bump-Steer at lowered car
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Thread: Bump-Steer at lowered car

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    3000 Post Club Site Supporter P.J. Romano's Avatar
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    Bump-Steer at lowered car

    Lowering the GT is popular modification. Aside of making car look cool, it greatly improves handling. Unfortunately, lowering the car also causes increase of bump-steer effect. Bump-steer is the term for the tendency of the wheel of a car to steer itself as it moves through the suspension stroke (Wikipedia). This is more of a problem on a street car comparing to racing car as the suspension moves up and down more.
    The reason for bump-steer at lowered car is changed position of steering tie rods. Optimally tie rods should be parallel to the lower control arms. On a lowered car tie rods point upward which disturbs optimal suspension parallelogram. To put it in simple words, the car gets twitchy on bumpy roads. My GT is lowered 2” (50 mm) by combined use of intermediate spring and longer lower ball joints and I was always joking how it keeps you busy when driving.

    Making it one of my winter projects, I decided to correct bump-steer by lowering position of tie rod ends. This is done by installing tie rods under the steering arms. Relatively easy project, with the only difficulty being now wrong orientation of the tapered hole for tie-rod end. I have fixed that by enlarging existing hole and installing steel bushing with “inverted” tapered hole for tie-rod end. Bushings were made by my buddy Earl in his machine shop. In order to prevent bushing to be pulled out of the arm, the bottom end is flared, similar to the head of countersink bolt. Bushing is pressed in the steering arm for a very tight fit.
    Pictures show difference in the position of tie rods before and after modification. Not perfectly parallel position but much smaller angle of the tie rod. Dry weather the other day allowed me to make short run after re-adjusting wheels toe-in. The ride was smooth and the car was noticeably less twitchy. I wish I have done this modification earlier.

    I should add that if the car is lowered just 1” (25 mm), there will likely be no need for such modification. 2” (50 mm) lowered car will much benefit if the tie rod position is changed as described. Personally, I found 2” lowering spring way too hard for street use. Intermediate spring plus longer lower ball joint will lower the car 2” but will still keep the ride reasonably comfortable.
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    Old racers never die. They just go bench racing.

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    7,000 Post Club wrench459 (R.I.P.)'s Avatar
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    I've ran into bump steer on a quarter mile paper clip dirt track

    Si vis pacem, para bellum "If you want peace, prepare for war"

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    Über Genius First opel 1981's Avatar
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    You could accomplish the same thing by lowering the steering rack. Just saying.

    In the process, you could change out the steering rack for one from something with power steering. Just saying.
    Opel GTs are not GM products
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    3000 Post Club Site Supporter P.J. Romano's Avatar
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    It is doable, but installing the steering rack in higher (not lower) position would be larger project. The steering rack is bolted on the suspension cross member, so additional support plate must be fabricated extending above the cross member. Also, I already have the power steering (electrical).
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    Last edited by P.J. Romano; 12-09-2017 at 12:11 PM.
    Old racers never die. They just go bench racing.

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    Hoosier Opeler Site Supporter rrossjr's Avatar
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    P.J.
    Nice explanation of bump-steer (not to be confused with bum steer)
    and how to compensate for it in a lowered GT. Also, excellent use of
    "parallelogram" in a sentence!
    Cheers,
    Ron in Indy
    Gary, Ooooner, kwschumm and 2 others like this.

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    Correct bump steer is just as important as the rest of your front end geometry. The outer tie rod end must approximate the same arc of travel as the moment center at the base of the spindle as it travels upward from ride height or it will be downright dangerous to drive. Bump Steer Gauges are available from several sources like Mittler Bros, and Longacres Racing Products. They employ the use a pair of precision dial indicators placed on a stand at the width of the tread. A wide, graduated plate rides on the face of the hub to get a very accurate representation of how far the tie rod "pulls" on the steering knuckle under 2" or so of bump and rebound. Many racers opt to replace the outer tie rod end with a HD heim joint fastened with GR8 hardware through the steering knuckle. Spacers between the two will raise and lower the arc center accordingly for fine tuning. Proper clearance inside the wheel can sometimes be the limiting factor. This is often overlooked when guys are setting up their race cars. Just another one of those little secrets to going fast.
    rrossjr likes this.

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    PJ,
    Do you happen to know the actual vertical difference of above vs. below mounting? If the target was 1.0" lower (your ball joint lengthening), it appears to me that this modification lowers the outer tie rod ~2.0" so you'll have a similar problem, just from the other side.

    The more I look into it, the base Opel suspension wasn't too bad, other than the coil bind and flexible arms. The geometry could use a slightly higher roll center, but as you've shown that can create other issues.

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    3000 Post Club Site Supporter P.J. Romano's Avatar
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    The target was full 2" (Intermediate spring + longer ball joint). The pictures are somewhat deceiving as I had to lay on the ground to take the photos. My car sits quite low.
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    Last edited by P.J. Romano; 12-11-2017 at 12:14 AM.
    Old racers never die. They just go bench racing.

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    A nice add comes on below when read the last post from P.J. and look at summer picture in Goderich

    Hey Canuck,its your fuel consumption
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    Quote Originally Posted by P.J. Romano View Post
    The target was full 2" (Intermediate spring + longer ball joint). The pictures are somewhat deceiving as I had to lay on the ground to take the photos. My car sits quite low.
    To my understanding, the longer ball joint will changing in the tie rod geometry but the lower spring will just exercise the production suspension though its normal range, with not much compensation required. It seems the goal is to have the tie rod almost parallel with the lower arm, but technically pointing to the instant center from the LCA and UCA.
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    Project 1450 supporter... Site Supporter RallyBob's Avatar
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    PJ, one of my front suspension builds was in need of some serious bump steer and Ackerman correction. I did the taller ball joints, using low friction +1/2" units (about 1-3/8" taller than the original Opel ones), and I also used lowering springs. I wanted to change the Ackerman, and this in conjunction with the increased castor I added really messed up the bump steer curve.

    My solution was a modified steering arm with spacers to mount it to the spindle (this is a Manta front end BTW). I also made my own outer tie rods with high misalignment monoballs, as in the past I have destroyed OEM outer tie rods rapidly due to high cornering loads and wide tires. This allowed me to use a thru-bolt and shims to get the exact bump steer curve I wanted.

    At the same time, I made the steering arm a bit shorter to quicken the steering ratio a bit. It's now 2.15 turns, lock-to-lock.








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    3000 Post Club Site Supporter P.J. Romano's Avatar
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    Holy cow, Bob!

    Sometimes the theory and seat-of-pants feedback do not agree. The problem is poorly designed GT/Kadett suspension from the begining where the ideal parallelogram does not exist. The picture from Service manual few posts ago shows that steering rods are far from being parallel to the lower arms. Other than doing major redesign, my modification has brought the steering rods to better angle although not perfect.

    During short test rides before the snow, the car felt much better than before. I will do more testing when the weather allows.
    Last edited by P.J. Romano; 12-11-2017 at 12:10 PM.
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    Old racers never die. They just go bench racing.

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    Tennessean Site Supporter hrcollinsjr's Avatar
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    Quote Originally Posted by P.J. Romano View Post
    During short test rides before the snow, the car felt much better than before. I will do more testing when the weather allows.
    Probably a dumb question but, have you had it realigned since doing your modification?

    Harold

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    3000 Post Club Site Supporter P.J. Romano's Avatar
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    Harold!
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    Old racers never die. They just go bench racing.

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    Tennessean Site Supporter hrcollinsjr's Avatar
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    Quote Originally Posted by P.J. Romano View Post
    Harold!
    Sorry I asked.

    Harold
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    7,000 Post Club wrench459 (R.I.P.)'s Avatar
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    Quote Originally Posted by hrcollinsjr View Post
    Probably a dumb question but, have you had it realigned since doing your modification?

    Harold
    Is there a difference between EPS vs Manual specs?

    Si vis pacem, para bellum "If you want peace, prepare for war"

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    Opeler
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    Hi There!

    New to the forum but very interested in extended ball joints for my Opel Kadett. Where can I get a pair?

    THanks

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    Project 1450 supporter... Site Supporter RallyBob's Avatar
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    Quote Originally Posted by avendler View Post
    Hi There!

    New to the forum but very interested in extended ball joints for my Opel Kadett. Where can I get a pair?

    THanks
    There is no bolt-in solution.

    Most of us use Chrysler upper ball joints as lowers in a GT/Kadett/Manta.

    But it requires machining for...and welding in, a threaded sleeve for the ball joint. And in the case of a GT or Kadett, machining the lower spindle taper to a larger ID, but still retaining the 7 degree taper.
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    Last edited by RallyBob; 03-04-2019 at 03:40 PM.
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    '70 GT 'Bonnie', '71 Ascona 4-dr turbo - winter beater, '71 Ascona 4-dr 'Turd' - rallycar, '72 Ascona wagon - 'Red', '72 Manta - caged street car, '74 Manta Luxus - factory sunroof, '74 Manta ITB racecar, '75 Manta - racecar

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    Member Mercougary's Avatar
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    Quote Originally Posted by RallyBob View Post
    There is no bolt-in solution.

    Most of us use Chrysler upper ball joints as lowers in a GT/Kadett/Manta.

    But it requires machining for...and welding in, a threaded sleeve for the ball joint. And in the case of a GT or Kadett, machining the lower spindle taper to a larger ID, but still retaining the 7 degree taper.
    Nice work Bob. I like your positive stops for micro adjustments in the camber as well. Do you have the part number for that particular upper ball joint? And, the weldable threaded sleeve appears to be Speedway Eng circle track part. Is that correct?

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    Thanks for the info. I suspected that those might be the right part. I see them available from JEGS and other circle track suppliers. I have the capability to make that setup so I'll get on it soon. I have a 24HoursOfLemons race this weekend so the car is going as it is now but after that I'll do the Chrysler ball joints. BTW, here is a pic of my through the frame front swaybar setup. Works really well. It's a 28" long 7/8"spline nascar rear sway with an off the shelf aluminum arm. I used a pair of bulkhead mount bearing plates welded to the framerail. All these parts are cheap on ebay.
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